Doctors are aware of any potential risk of X-rays, especially in a growing child. They also keep in mind the cost of additional imaging studies. If they suggest X-rays at regular intervals, it’s usually for a specific reason.
Most likely the type of fracture your daughter has requires careful management. Poor healing or healing in a deformed position will be trouble later. X-rays can show if the cast is too loose and needs to be replaced. Any angle of deformity will also show up.
Nonunion or too much deformity may be a signal that further treatment is needed. Sometimes surgery is needed to further reduce or stabilize the fracture. Reduction means to line the bones up evenly for optimal healing. If the cast isn’t enough to do this, then wires, screws, or a metal plate may be needed.
The surgeon may also be treating your daughter with a slightly different plan of care based on her involvement in gymnastics. Good function and strength needed for back flips and other types of gymnastic activities requires healing in a good position.